U.S. patent application number 11/534587 was filed with the patent office on 2008-03-27 for apparatus and method for providing protection for a synchronous electrical generator in a power system.
Invention is credited to Hector J. Altuve-Ferrer, Armando Guzman-Casillas.
Application Number | 20080074810 11/534587 |
Document ID | / |
Family ID | 39224685 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080074810 |
Kind Code |
A1 |
Guzman-Casillas; Armando ;
et al. |
March 27, 2008 |
APPARATUS AND METHOD FOR PROVIDING PROTECTION FOR A SYNCHRONOUS
ELECTRICAL GENERATOR IN A POWER SYSTEM
Abstract
An apparatus and method provide protection for a synchronous
generator in a power system. The method includes deriving a
plurality of generator safe operating boundary data expressions
from power system data and/or user-defined inputs. The power system
data may include a plurality of generator data supplied by a
manufacturer of the generator and/or power system parameters such
as power system equivalent impedance. Each generator safe operating
boundary data expression may relate to a generator capability
curve, a steady-state stability limit curve, a minimum excitation
limiter curve, an over excitation limiter curve, or an user-defined
curve. The method also includes calculating an active power value
sum and a reactive power value sum based on generator three-phase
currents and voltages, comparing these sums to at least one of the
plurality of generator safe operating boundary data expressions,
and to provide protection and/or alarming functions for the
generator based on this comparison.
Inventors: |
Guzman-Casillas; Armando;
(Pullman, WA) ; Altuve-Ferrer; Hector J.;
(Monterrey, MX) |
Correspondence
Address: |
COOK, ALEX, MCFARRON, MANZO, CUMMINGS & MEHLER LTD
SUITE 2850, 200 WEST ADAMS STREET
CHICAGO
IL
60606
US
|
Family ID: |
39224685 |
Appl. No.: |
11/534587 |
Filed: |
September 22, 2006 |
Current U.S.
Class: |
361/20 |
Current CPC
Class: |
H02P 9/006 20130101;
H02H 7/065 20130101 |
Class at
Publication: |
361/20 |
International
Class: |
H02H 7/06 20060101
H02H007/06 |
Claims
1. A method for providing synchronous generator protection using a
protective relay, the method comprising: deriving a plurality of
generator safe operating boundary data expressions from a plurality
of power system data; and utilizing at least one of the plurality
of generator safe operating boundary data expressions to provide
the protection for a synchronous generator.
2. The method of claim 1, further comprising the step of providing
to the protective relay coordinates of a loss-of-field element
characteristic and power system data comprising coordinates of a
capability curve.
3. The method of claim 2, further comprising the step of selecting
between a straight line configuration and a curved line
configuration for one of the generator safe operating boundary data
expressions.
4. The method of claim 1, wherein, for automatic determination of
the generator safe operating boundary data expressions by the
protective relay, the plurality of power system data comprises:
coordinates of a capability curve; an impedance of the synchronous
generator; and power system parameters.
5. The method of claim 4, wherein the power system parameters
comprises equivalent system impedance.
6. The method of claim 1, wherein each of the plurality of
generator safe operating boundary data expressions is at least one
selected from the group consisting of quadratic equations, circle
equations, look-up tables and linear equations.
7. The method of claim 1, wherein each of the plurality of power
system data is set in relation to at least one selected from the
group consisting of a generator capability curve, a minimum
excitation limiter curve, and an over excitation limiter curve.
8. The method of claim 1, wherein the generator safe operating
boundary data expressions comprise an expression for defining at
least one selected from the group consisting of: a boundary for
field winding heating associated with a field winding current
limit, a boundary for armature heating associated with an armature
current limit, a boundary for stator core temperature associated
with a stator end region heating limit, a steady-state stability
limit curve, a minimum excitation limiter curve, an over excitation
limiter curve, and combinations thereof.
9. The method of claim 8, wherein the boundary for field winding
heating associated with a field winding current limit is defined by
the following expression: S ( .beta. ) = R e i .beta. + i C for
.rho. .ltoreq. .beta. .ltoreq. .pi. 2 ##EQU00007## Where: R is the
radius of the circle C is the center of the circle .rho. is the
circle lower limit
10. The method of claim 8, wherein the boundary for armature
heating associated with an armature current limit is defined by the
following expression: S(.beta.)=Re.sup.i.beta.+iC for
-.alpha..ltoreq..beta..ltoreq..phi. Where: R is the radius of the
circle C is the center of the circle .phi. is the circle upper
limit that corresponds to the minimum lagging power factor -.alpha.
is the circle lower limit that corresponds to the minimum leading
power factor
11. The method of claim 8, wherein the boundary for stator core
temperature associated with a stator end region heating limit is
defined by the following expression: S ( .beta. ) = R e i .beta. +
i C for 3 2 .pi. .ltoreq. .beta. .ltoreq. - .gamma. ##EQU00008##
Where: R is the radius of the circle C is the center of the circle
-.gamma. is the circle upper limit
12. The method of claim 1, further comprising: providing user
programmable inputs to the protective relay; and providing
generator operating indications to the protective relay; the user
programmable inputs and the generator operating indications
determining selection of the at least one of the plurality of
generator safe operating boundary data expressions utilized to
provide protection for the synchronous generator.
13. The method of claim 1, further comprising: based on measured
three-phase currents and voltages associated with synchronous
generator operation, calculating an active power value sum and a
reactive power value sum; comparing the active power value sum and
a reactive power value sum to the at least one of the plurality of
generator safe operating boundary data expressions; and based on
the comparison, providing the protection for the synchronous
generator.
14. The method of claim 1 wherein the generator safe operating
boundary data expressions comprise a loss-of-field element
characteristic.
15. The method of claim 14 further including the step of
calculating a steady-state stability limit (SSSL) curve and wherein
the loss-of-field characteristic is situated with respect to the
SSSL curve.
16. The method of claim 15 wherein the loss-of-field characteristic
is situated with respect to the stator end region limit curve.
17. The method of claim 1 further including the step of providing
an undervoltage element.
18. The method of claim 1 further including the step of providing
one or more active power elements.
19. The method of claim 17 wherein the undervoltage element
accelerates a trip or alarm assertion when a low voltage condition
indicates that the power system may collapse.
20. The method of claim 18 wherein at least one of the active power
elements is adaptable to the load condition of the generator.
21. The method of claim 13 further including calculating either the
active power value sum or the reactive power value sum using
positive-sequence voltage and positive-sequence current values.
22. The method of claim 21 wherein the calculation of the active
power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00009## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00010## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence active
power equals: P.sub.1=real(S.sub.1) (4) Where:
a:=e.sup.j120.degree.
23. The method of claim 21 wherein the calculation of the reactive
power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00011## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00012## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence
reactive power equals: Q.sub.1=imag(S.sub.1) (4) Where:
a:=e.sup.j120.degree.
24. In a protective relay, a method for providing synchronous
generator protection, the method comprising: selecting at least one
of a plurality of generator safe operating boundary data
expressions based on a generator operating indication and a
predetermined user programmable input, the plurality of generator
safe operating boundary data expressions derived from a plurality
of power system data; calculating an active power value sum and a
reactive power value sum based on measured three-phase currents and
voltages associated with synchronous generator operation; comparing
the active power value sum and a reactive power value sum to the at
least one of the plurality of generator safe operating boundary
data expressions; and providing the protection for a synchronous
generator based on the comparison.
25. The method of claim 24, further comprising the step of
providing to the protective relay coordinates of a loss-of-field
element characteristic and power system data comprising coordinates
of a capability curve.
26. The method of claim 24, further comprising the step of
selecting between a straight line configuration and a curved line
configuration for one of the generator safe operating boundary data
expressions.
27. The method of claim 24, wherein, for automatic determination of
the generator safe operating boundary data expressions by the
protective relay, the plurality of power system data comprises:
coordinates of a capability curve; an impedance of the synchronous
generator; and power system parameters.
28. The method of claim 27, wherein the power system parameters
comprises equivalent system impedance.
29. The method of claim 24, further comprising actuating an alarm
indication if the active and reactive power value sums are
determined to be outside of a generator normal operation zone based
on the comparison.
30. The method of claim 24, further comprising actuating a trip
signal if the active and reactive power values are determined to be
inside a protection zone based on the comparison.
31. The method of claim 24, wherein calculating the active power
value sum and the reactive power value sum comprises: calculating
an A-phase active power value and an A-phase reactive power value;
calculating a B-phase active power value and a B-phase reactive
power value; calculating a C-phase active power value and a C-phase
reactive power value; adding the A-phase active power value, the
B-phase active power value and the C-phase active power value to
form the active power value sum; and adding the A-phase reactive
power value, the B-phase reactive power value and the C-phase
reactive power value to form the reactive power value sum.
32. The method of claim 24, wherein each of the plurality of
generator safe operating boundary data expressions is at least one
selected from the group consisting of quadratic equations, circle
equations, look-up tables and linear equations.
33. The method of claim 24, wherein the generator safe operating
boundary data expressions comprise an expression for defining at
least one selected from the group consisting of: a boundary for
field winding heating associated with a field winding current
limit, a boundary for armature heating associated with an armature
current limit, a boundary for stator core temperature associated
with a stator end region heating limit, a steady-state stability
limit curve, a minimum excitation limiter curve, an over excitation
limiter curve, and combinations thereof.
34. The method of claim 33, wherein the boundary for field winding
heating associated with a field winding current limit is defined by
the following expression: S ( .beta. ) = R e i .beta. + i C for
.rho. .ltoreq. .beta. .ltoreq. .pi. 2 ##EQU00013## Where: R is the
radius of the circle C is the center of the circle .rho. is the
circle lower limit
35. The method of claim 33, wherein the boundary for armature
heating associated with an armature current limit is defined by the
following expression: S(.beta.)=Re.sup.i.beta.+iC for
-.alpha..ltoreq..beta..ltoreq..phi. Where: R is the radius of the
circle C is the center of the circle .phi. is the circle upper
limit that corresponds to the minimum lagging power factor -.alpha.
is the circle lower limit that corresponds to the minimum leading
power factor
36. The method of claim 33, wherein the boundary for stator core
temperature associated with a stator end region heating limit is
defined by the following expression: S ( .beta. ) = R e i .beta. +
i C for 3 2 .pi. .ltoreq. .beta. .ltoreq. - .gamma. ##EQU00014##
Where: R is the radius of the circle C is the center of the circle
-.gamma. is the circle upper limit
37. The method of claim 24, wherein each of the plurality of
generator safe operating boundary data expressions is at least one
selected from the group consisting of a generator capability curve,
a minimum excitation limiter curve, and an over excitation limiter
curve.
38. The method of claim 24, wherein the predetermined user
programmable input comprises a plurality of predetermined user
programmable inputs, each of the predetermined programmable inputs
associated with a different generator safe operating boundary data
expression of the plurality of generator safe operating boundary
data expressions utilized to provide generator protection for the
synchronous generator.
39. The method of claim 24 wherein the generator safe operating
boundary data expressions comprise a loss-of-field element
characteristic.
40. The method of claim 39 further including the step of
calculating a steady-state stability limit (SSSL) curve and wherein
the loss-of-field characteristic is situated with respect to the
SSSL curve.
41. The method of claim 40 wherein the loss-of-field characteristic
is situated with respect to the stator end region limit curve.
42. The method of claim 24 further including the step of providing
an undervoltage element.
43. The method of claim 24 further including the step of providing
one or more active power elements.
44. The method of claim 42 wherein the undervoltage element
accelerates a trip or alarm assertion when a low voltage condition
indicates that the power system may collapse.
45. The method of claim 43 wherein at least one of the active power
elements is adaptable to the load condition of the generator.
46. The method of claim 24 wherein the active power value sum and
the reactive power value sum are calculated using positive-sequence
voltage and positive-sequence current values.
47. The method of claim 46 wherein the calculation of the active
power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00015## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00016## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence active
power equals: P.sub.1=real(S.sub.1) (4) Where:
a:=e.sup.j120.degree.
48. The method of claim 46 wherein the calculation of the reactive
power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00017## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00018## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence
reactive power equals: Q.sub.1=imag(S.sub.1) (4) Where:
a:=e.sup.j120.degree.
49. An apparatus for providing synchronous generator protection,
the apparatus comprising: a means for deriving a plurality of
digitized signals representative of measured three-phase secondary
currents and voltages associated with synchronous generator
operation; and a microcontroller operatively coupled to the means
for deriving the plurality of digitized signals, the
microcontroller having a microprocessor and a memory operatively
coupled to the microprocessor, the microcontroller being programmed
to: based on a plurality of predetermined user programmable inputs
and generator operating indications, select at least one of a
plurality of generator safe operating boundary data expressions
derived from a plurality of power system data; calculate an active
power value sum and a reactive power value sum based on the
plurality of phasors; compare the active power value sum and the
reactive power value sum to the at least one of the plurality of
generator safe operating boundary data expressions; and provide the
protection for a synchronous generator based on the comparison.
50. The apparatus of claim 49, wherein the microcontroller is
further programmed to actuate an alarm indication if the active and
reactive power value sums are determined to be outside of a
generator normal operation zone based on the comparison.
51. The apparatus of claim 50, further comprising a timer
configured to delay the alarm indication by a preset time
period.
52. The apparatus of claim 49, wherein the microcontrollers is
further programmed to actuate a trip signal if the active and
reactive power value sums are determined to be inside a protection
zone based on the comparison.
53. The apparatus of claim 52, further comprising a timer
configured to delay the trip signal by a preset time period.
54. The apparatus of claim 49, wherein each of the plurality of
generator safe operating boundary data expressions is at least one
selected from the group consisting of quadratic equations, circle
equations, look-up tables and linear equations.
55. The apparatus of claim 49, wherein each of the plurality of
generator safe operating boundary data expressions is at least one
selected from the group consisting of a generator capability curve,
a minimum excitation limiter curve, and an over excitation limiter
curve.
56. The apparatus of claim 49, wherein the microcontroller is
adapted to derive the plurality of generator safe operating
boundary data expressions from generator impedance data.
57. The apparatus of claim 49, wherein the microcontroller is
adapted to derive the plurality of generator safe operating
boundary data expressions from equivalent power system impedance
data.
58. The apparatus of claim 49, wherein the generator safe operating
boundary data expressions comprises an expression for defining at
least one selected from the group consisting of: a boundary for
field winding heating associated with a field winding current
limit, a boundary for armature heating associated with an armature
current limit, a boundary for stator core temperature associated
with a stator end region heating limit, a steady-state stability
limit curve, a minimum excitation limiter curve, an over excitation
limiter curve, and combinations thereof.
59. The apparatus of claim 58, wherein the boundary for field
winding heating associated with a field winding current limit is
defined by the following expression: S ( .beta. ) = R e i .beta. +
i C for .rho. .ltoreq. .beta. .ltoreq. .pi. 2 ##EQU00019## Where: R
is the radius of the circle C is the center of the circle .rho. is
the circle lower limit
60. The apparatus of claim 58, wherein the boundary for armature
heating associated with an armature current limit is defined by the
following expression: S(.beta.)=Re.sup.i.beta.+iC for
-.alpha..ltoreq..beta..ltoreq..phi. Where: R is the radius of the
circle C is the center of the circle .phi. is the circle upper
limit that corresponds to the minimum lagging power factor -.alpha.
is the circle lower limit that corresponds to the minimum leading
power factor
61. The apparatus of claim 58, wherein the boundary for stator core
temperature associated with a stator end region heating limit is
defined by the following expression: S ( .beta. ) = R e i .beta. +
i C for 3 2 .pi. .ltoreq. .beta. .ltoreq. - .gamma. ##EQU00020##
Where: R is the radius of the circle C is the center of the circle
-.gamma. is the circle upper limit
62. The apparatus of claim 49 wherein the generator safe operating
boundary data expressions comprise a loss-of-field element
characteristic.
63. The apparatus of claim 62 further adapted to calculate a
steady-state stability limit (SSSL) curve and wherein the
loss-of-field characteristic is situated with respect to the SSSL
curve.
64. The apparatus of claim 63 wherein the loss-of-field
characteristic is situated with respect to the stator end region
heating limit curve.
65. The apparatus of claim 49 further including an undervoltage
element.
66. The apparatus of claim 49 further including the step of
providing one or more active power elements.
67. The apparatus of claim 65 wherein the undervoltage element
accelerates a trip or alarm assertion when a low voltage condition
indicates that the power system may collapse.
68. The apparatus of claim 66 wherein at least one of the active
power elements is adaptable to the load condition of the
generator.
69. The apparatus of claim 49 wherein the active power value sum
and the reactive power value sum are calculated using
positive-sequence voltage and positive-sequence current values.
70. The apparatus of claim 69 wherein the calculation of the active
power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00021## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00022## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence active
power equals: P.sub.1=real(S.sub.1) (4) Where:
a:=e.sup.j120.degree.
71. The apparatus of claim 69 wherein the calculation of the
reactive power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00023## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00024## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence
reactive power equals: Q.sub.1=imag(S.sub.1) (4) Where:
a:=e.sup.j120
72. A computer readable medium having program code recorded thereon
for providing synchronous generator protection comprising; a first
program code for selecting at least one of a plurality of generator
safe operating boundary data expressions based on a generator
operating indication and a predetermined user programmable input,
the plurality of generator safe operating boundary data expressions
derived from a plurality of power system data; a second program
code for calculating an active power value sum and a reactive power
value sum based on measured three-phase currents and voltages
associated with synchronous generator operation; a third program
code for comparing the active power value sum and a reactive power
value sum to the at least one of the plurality of generator safe
operating boundary data expressions; and a fourth program code for
providing the protection for a synchronous generator based on the
comparison.
73. The program code of claim 72, wherein each of the plurality of
generator safe operating boundary data expressions is at least one
selected from the group consisting of quadratic equations, circle
equations, look-up tables and linear equations.
74. The program code of claim 72, wherein each of the plurality of
generator safe operating boundary data expressions is at least one
selected from the group consisting of a generator capability curve,
a minimum excitation limiter curve, and an over excitation limiter
curve.
75. The program code of claim 72, wherein the generator safe
operating boundary data expressions comprises an expression for
defining at least one selected from the group consisting of: a
boundary for field winding heating associated with a field winding
current limit, a boundary for armature heating associated with an
armature current limit, a boundary for stator core temperature
associated with a stator end region heating limit, a steady-state
stability limit curve, a minimum excitation limiter curve, an over
excitation limiter curve, and combinations thereof.
76. The program code of claim 75, wherein the boundary for field
winding heating associated with a field winding current limit is
defined by the following expression: S ( .beta. ) = R e i .beta. +
i C for .rho. .ltoreq. .beta. .ltoreq. .pi. 2 ##EQU00025## Where: R
is the radius of the circle C is the center of the circle .rho. is
the circle lower limit
77. The program code of claim 75, wherein the boundary for armature
heating associated with an armature current limit is defined by the
following expression: S(.beta.)=Re.sup.i.beta.+iC for
-.alpha..ltoreq..beta..ltoreq..phi. Where: R is the radius of the
circle C is the center of the circle .phi. is the circle upper
limit that corresponds to the minimum lagging power factor -.alpha.
is the circle lower limit that corresponds to the minimum leading
power factor
78. The program code of claim 75, wherein the boundary for stator
core temperature associated with a stator end region heating limit
is defined by the following expression: S ( .beta. ) = R e i .beta.
+ i C for 3 2 .pi. .ltoreq. .beta. .ltoreq. - .gamma. ##EQU00026##
Where: R is the radius of the circle C is the center of the circle
-.gamma. is the circle upper limit
79. The program code of claim 72 wherein the generator safe
operating boundary data expressions comprise a loss-of-field
characteristic.
80. The program code of claim 79, wherein the predetermined user
programmable input comprises coordinates of the loss-of-field
characteristic.
81. The program code of claim 72, wherein power system data
comprise coordinates of a capability curve, an impedance of the
generator, and power system parameters.
82. The program code of claim 80, wherein the power system
parameters comprises equivalent power system impedance.
83. The program code of claim 79 further for calculating a
steady-state stability limit (SSSL) curve and wherein the
loss-of-field characteristic is situated with respect to the SSSL
curve.
84. The program code of claim 83 wherein the loss-of-field
characteristic is situated with respect to the stator end region
heating limit curve.
85. The program code of claim 72 wherein the active power value sum
and the reactive power value sum are calculated using
positive-sequence voltage and positive-sequence current values.
86. The program code of claim 72 wherein the calculation of the
active power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00027## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00028## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence active
power equals: P.sub.1=real(S.sub.1) (4) Where:
a:=e.sup.j120.degree.
87. The program code of claim 72 wherein the calculation of the
reactive power value sum is defined by the following expressions:
Positive-sequence voltage equals: V 1 = V A + a V B + a 2 V C 3 ( 1
) ##EQU00029## Positive-sequence current equals: I 1 = I A + a I B
+ a 2 I C 3 ( 2 ) ##EQU00030## Positive-sequence apparent power
equals: S.sub.1=3V.sub.1conj(I.sub.1) (3) Positive-sequence
reactive power equals: Q.sub.1=imag(S.sub.1) (4) Where:
a:=e.sup.j120.degree.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] None
BACKGROUND OF THE INVENTION
[0002] The present invention generally relates to synchronous
generators, and more specifically, to an apparatus and method for
providing generator protection for a synchronous electrical
generator in a power system.
[0003] Synchronous electrical generators ("synchronous generators")
are used in many applications requiring alternating current (AC)
power generation. For example, electric utility systems or power
systems include a variety of power system elements such as
synchronous generators, power transformers, power transmission
lines, distribution lines, buses, capacitors, etc. to generate,
transmit and distribute electrical energy to loads. A synchronous
generator operates to, for example, convert mechanical rotation via
a prime mover (e.g., shaft rotation provided by a coal powered
steam turbine) into AC current via electromagnetic principles.
After suitable conditioning, the alternating electrical current is
transmitted and distributed as three-phase electric power to a
variety of loads.
[0004] As is known, synchronous generator design is based on
Faraday's law of electromagnetic induction and includes a
rotational portion for inducing an electromotive force (EMF) in a
stationary portion. The rotational portion is driven by the prime
mover. More specifically, the rotational portion, or rotor,
includes a field winding wrapped around a rotor body, and the
stationary portion includes a stator having an armature winding.
The rotor body, typically made of steel, may have a salient pole
structure (i.e., poles protruding from a shaft) or a cylindrical
structure.
[0005] In operation, EMFs are induced in the armature windings of
the stator upon application of DC current to the field winding of
the rotor. That is, direct current is made to flow in the field
winding. This results in a magnetic field, and when the rotor is
made to rotate at a constant speed, the magnetic field rotates with
it. Accordingly, as the moving magnetic field passes through the
stator winding(s), an EMF is induced therein. If the stationary
armature includes, for example, three stationary armature windings,
they experience a periodically varying magnetic field, and three
EMFs are induced therein. These three EMFs conform a three-phase
system of voltages. Thus, for 60-Hz AC systems, in a two-pole
machine, the rotor has to rotate at 3600 revolutions per minute
with three armature windings displaced equally in space on the
stator body to generate three-phase electric power.
[0006] As the generator electric load increases, the generator
demands more mechanical power from its prime mover and more current
flows through the stator winding, and therefore more electric
active power is delivered from the synchronous generator to the
power system. By increasing the current to the rotor winding, the
synchronous generator produces more reactive power, also called
reactive volt-amperes (VARs), which, in effect, can raise the power
system voltage. Conversely, by decreasing the current in the rotor
winding, VARs are absorbed by the generator, effectively lowering
the power system voltage. As is known, we express in Watts or
Megawatts the active power delivered to or consumed by a load,
while VAR or MVAR is the imaginary counterpart of the Watt or
Megawatt and represents the reactive power consumed or generated by
a reactive load (i.e., a load having a phase difference between the
applied voltage and the current).
[0007] Generator capability curves ("capability curves") are
typically provided by a generator manufacturer to define the
operating or thermal limits of a particular synchronous generator
at different cooling pressures. Each capability curve represents
the synchronous generator capability limit for a pressurized
coolant (e.g., hydrogen) circulating to cool the stator and rotor
windings. More cooling enables more armature current to flow during
synchronous generator operation, while less cooling enables less
current to flow. Additionally, over excitation limiter (OEL) curves
and minimum excitation limiter (MEL) curves are typically included
with the manufacturer-provided capability curves. Steady state
stability limit (SSSL) curves may further be determined with
generator impedance data and power system parameters.
[0008] Because there are limits to the amount of current that can
flow through the stator and rotor winding, the operating limits
reflected in the capability curves are imposed on the amount of
Watts and VARs that the synchronous generator can deliver to the
power system. There is also a minimum value of current that must
flow in the rotor field to maintain generator stability, and this
imposes a limit on the amount of VARs that the synchronous
generator can absorb for each delivered active power value. Thus,
the operating limits graphically illustrated by the capability
curve(s) include an active power component "P" expressed in
Megawatts (MW) and a reactive power component "Q" expressed in Mega
VARs (MVARs). As long as the P, Q operating point of the
synchronous generator (i.e., as long as the amount of Watts and
VARs flowing out of or into the generator) is within its safe
operating limits, or inside its capability curve, the synchronous
generator will operate within safe limits.
[0009] Although the operating limits defined by capability curves
are utilized by power generating station operators to ensure safe
synchronous generator operation, it has been suggested to utilize
these curves to influence excitation control of a synchronous
generator in real time. For example, U.S. Pat. No. 5,264,778,
entitled "Apparatus Protecting a Synchronous Machine from Under
Excitation," issued on Nov. 23, 1993, describes a microprocessor
based voltage regulator system that provides a minimum limit on
excitation that is defined using one or more straight line segments
approximating the associated machine capability curves. Such a
minimum limit on excitation prevents the excitation of the
synchronous generator from falling below a predetermined P-Q
characteristic. The microprocessor based voltage regulator system
of the U.S. Pat. No. 5,264,778 is included in a control system of
the synchronous generator.
[0010] It has also been suggested that synchronous generator
operation may be improved via use of a visual display that reflects
synchronous generator operation with respect to its capability
curves. U.S. Pat. No. 5,581,470, entitled "Apparatus for Visually
Graphically Displaying the Generator Point of a Generator in
Reference to its Capability Curve Including Digital Readouts of
Watts, VARs and Hydrogen Pressure," issued on Dec. 3, 1996,
describes a computer-based meter that provides a real time
graphical display which visually indicates an operating point in
relation to a capability curve(s) of a synchronous generator during
operation. The operating point(s) and capability curves are defined
and displayed based on measurement signals from Watt, VAR and
hydrogen pressure transducers.
[0011] Synchronous generator outages or failures due to power
system faults, abnormal operating conditions, and the like, can be
some of the costliest in the power system. Accordingly, protective
devices are operatively coupled to the synchronous generators and
their outputs in order to measure currents and voltages indicative
of synchronous generator operation. Such protective devices are
referred to hereinafter as protective relays, and typically include
a variety of protective functions or elements.
SUMMARY OF THE INVENTION
[0012] According to an embodiment of the invention, a method
enables protection for a synchronous generator. The method includes
deriving a plurality of generator safe operating boundary data
expressions from a plurality of power system data. The power system
data may be supplied by a manufacturer of the synchronous
generator. The power system data may include generator impedance.
The power system data may include power system parameters such as
equivalent power system impedance. The generator safe operating
boundary data expressions may be used by a protective relay. During
operation, the protective relay utilizes at least one of the
plurality of generator safe operating boundary data expressions to
enable the protection for the synchronous generator. Each of the
plurality of generator safe operating boundary data expressions is
selected from the group consisting of quadratic equations, circle
equations, look-up tables, linear equations and combinations
thereof.
[0013] The generator safe operating boundary data expressions may
be set in relation to a generator capability curve, a steady-state
stability limit curve, a minimum excitation limiter curve, or an
over excitation limiter curve. It is contemplated that the user may
manually set the generator safe operating boundary data expressions
(e.g., in relation to a manufacturer provided generator capability
curve, by providing generator safe operating boundaries such as the
capability curve and user-defined loss-of-field element) or, in
some cases, the generator safe operating boundary data expressions
may be derived based on generator and/or power system operating
limits (e.g., in relation to a steady-state stability limit curve,
a minimum excitation limiter curve, or an over excitation limiter
curve). In one embodiment, the protective relay determines the
generator safe operating boundary data expressions automatically
using generator capability curve data, generator impedance data and
power system impedance data.
[0014] According to another embodiment of the invention, the
generator safe operating boundary data expression is set for
loss-of-field protection. In one example, a loss-of-field element
is provided which is set with respect to the generator capability
curve. In another example, a loss-of-field element is provided
which is set with respect to an SSSL curve. In another embodiment,
one or more active power elements, and/or an undervoltage element
may further be provided.
[0015] According to another embodiment of the invention, in a
protective relay, a method provides protection for a synchronous
generator. The method includes selecting at least one of a
plurality of generator safe operating boundary data expressions
based on a predetermined user programmable input and a generator
operating indication. The plurality of generator safe operating
boundary data expressions is derived from either an SSSL curve, a
plurality of generator safe operating boundaries supplied by a
manufacturer of the synchronous generator, or other similar means.
The method also includes calculating an active power value sum and
a reactive power value sum based on measured three-phase currents
and voltages associated with synchronous generator operation. The
method further includes comparing the active power value sum and
the reactive power value sum to the at least one of the plurality
of generator safe operating boundary data expressions, and
providing the protection for the synchronous generator based on
this comparison.
[0016] An apparatus provides protection for a synchronous generator
in a power system. The apparatus comprises a means for deriving a
plurality of digitized signals representative of measured
three-phase secondary currents and voltages associated with
synchronous generator operation, and a microcontroller operatively
coupled to the means for deriving the plurality of digitized
signals. The microcontroller includes a microprocessor and a memory
operatively coupled to the microprocessor. The microcontroller is
programmed to, based on a plurality of predetermined user
programmable inputs and generator operating indications, select at
least one of a plurality of generator safe operating boundary data
expressions. The plurality of generator safe operating boundary
data expressions may be derived from a plurality of generator safe
operating boundaries supplied by a manufacturer of the synchronous
generator. The plurality of generator safe operating boundary data
expressions may be derived from user-defined values such as
coordinates of a loss-of-field element. The microcontroller is also
programmed to calculate an active power value sum and a reactive
power value sum based on the plurality of digitized signals,
compare the active power value sum and the reactive power value sum
to the at least one of the plurality of generator safe operating
boundary data expressions, and provide protection for the
synchronous generator based on this comparison.
[0017] A computer readable medium having program code recorded
thereon provides protection for a synchronous generator in a power
system. The computer readable medium includes a first program code
for selecting at least one of a plurality of generator safe
operating boundary data expressions based on a generator operating
indication and a predetermined user programmable input. The
plurality of generator safe operating boundary data expressions is
derived from a plurality of generator safe operating boundaries
supplied by a manufacturer of the synchronous generator. The
computer readable medium also includes a second program code for
calculating an active power value sum and a reactive power value
sum based on measured three-phase currents and voltages associated
with synchronous generator operation, a third program code for
comparing the active power value sum and reactive power value sum
to at least one of the plurality of generator safe operating
boundary data expressions, and a fourth program code for providing
the protection for the synchronous generator based on the
comparison.
[0018] It should be understood that the present invention includes
a number of different aspects and/or features which may have
utility alone and/or in combination with other aspects or features.
Accordingly, this summary is not an exhaustive identification of
each such aspect or feature that is now or may hereafter be
claimed, but represents an overview of certain aspects of the
present invention to assist in understanding the more detailed
description of preferred embodiments that follow. The scope of the
invention is not limited to the specific embodiments described
below, but is set forth in the claims now or hereafter filed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a single line schematic of a power system that may
be utilized in a typical wide area network.
[0020] FIG. 2a is an exemplary functional block of a generator
protective relay of FIG. 1, according to an embodiment of the
invention.
[0021] FIG. 2b is an exemplary functional block of the generator
operating boundary function of generator protection relay of FIG.
2a.
[0022] FIG. 2c is an exemplary functional block of the generator
operating boundary function adapted for providing positive-sequence
values of generator protection relay of FIG. 2a.
[0023] FIG. 3 is an exemplary set of generator capability curves
that may be provided by a synchronous generator manufacturer to
define the operating limits of a synchronous generator of FIG.
1.
[0024] FIG. 4 is one of the capability curves of the exemplary set
of generator capability curves of FIG. 3.
[0025] FIG. 5a is a flowchart of a method for synchronous generator
protection in the power system of FIG. 1, according to an
embodiment of the invention.
[0026] FIG. 5b is a flowchart of a method for asserting either an
alarm condition or trip condition for synchronous generator
protection in the power system of FIG. 1, according to an
embodiment of the invention.
[0027] FIG. 6 is a generator capability curve generated based on
generator safe operating boundary data expressions derived from the
capability curve of FIG. 4 for use by the generator protection
relay of FIG. 2a, according to an embodiment of the invention.
[0028] FIG. 7 is a graphical representation of an estimation curve
for the boundary for field winding heating associated with the
field winding current limit of a generator capability curve.
[0029] FIG. 8 is a graphical representation of an estimation curve
for the boundary for armature heating associated with the armature
current limit of a generator capability curve.
[0030] FIG. 9 is a graphical representation of an estimation curve
for the boundary for a stator core temperature associated with the
stator end region heating limit of a generator capability
curve.
[0031] FIG. 10 is a generator capability curve including an
arrangement for loss-of-field protection further illustrating the
protection zone whereupon a trip signal would be asserted if a
condition were to fall therein.
[0032] FIG. 11 is another generator capability curve including an
arrangement for loss-of-field protection further illustrating the
protection zone whereupon a trip signal would be asserted if a
condition were to fall therein.
[0033] FIG. 12 is a generator capability curve including an
arrangement for issuing an alarm signal and showing the alarming
zone.
[0034] FIG. 13 is a generator capability curve including an
arrangement for loss-of-field protection, and an arrangement for
issuing an alarm signal, and also showing the generator normal
operation zone, the protection zone, and the alarming zone.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] FIG. 1 is a single line schematic diagram of a power system
10 that may be utilized in a typical wide area system. As
illustrated in FIG. 1, the power system 10 includes, among other
things, three synchronous generators 11, 12 and 13, configured to
generate three-phase voltage sinusoidal waveforms such as 12 kV
sinusoidal waveforms, three step-up power transformers 14a, 14b and
14c, configured to increase the generated voltage sinusoidal
waveforms to higher voltage sinusoidal waveforms such as 138 kV
sinusoidal waveforms and a number of circuit breakers 18. The
step-up power transformers 14a, 14b, 14c operate to provide the
higher voltage sinusoidal waveforms to a number of long distance
transmission lines such as the transmission lines 20a, 20b and 20c.
In an embodiment, a first substation 16 may be defined to include
the two synchronous generators 11 and 12, the two step-up power
transformers 14a and 14b and associated circuit breakers 18, all
interconnected via a first bus 19. A second substation 35 may be
defined to include the synchronous generator 13, the step-up power
transformer 14c and associated circuit breakers 18, all
interconnected via a second bus 25. At the end of the long distance
transmission lines 20a, 20b, a third substation 22 includes two
step-down power transformers 24a and 24b configured to transform
the higher voltage sinusoidal waveforms to lower voltage sinusoidal
waveforms (e.g., 15 kV) suitable for distribution via one or more
distribution lines 26 to loads such as a load 32. The second
substation 35 also includes two step-down power transformers 24c
and 24d on respective distribution lines 28 and 29 to transform the
higher voltage sinusoidal waveforms, received via the second bus
25, to lower voltage sinusoidal waveforms suitable for use by
respective loads 30 and 34.
[0036] As discussed above, one or more protective relays are
operatively coupled to the synchronous generators 11, 12 and 13 to
measure currents and voltages indicative of synchronous generator
operation. Based on the measured currents and/or voltages, one or
more protective elements (e.g., an over-voltage element) of the
protective relay may operate to actuate a trip action in the event
of an abnormal condition. In the illustrated example of FIG. 1,
protection of the generator 12 is provided by a protective relay
100. While not separately shown, it should be understood that
additional protective relays 100 may be included in the power
system 10.
[0037] FIG. 2a is an exemplary functional block diagram of the
protective relay 100, according to an embodiment of the invention.
It should be understood that functional block diagram of FIG. 2a is
only one example of a protective relay implementation of the
instant invention, and that other implementations are possible.
[0038] As illustrated, the protective relay 100 includes a number
of inputs 101-106 configured to receive secondary current waveforms
I.sub.A, I.sub.B, and I.sub.C and secondary voltage waveforms
V.sub.A, V.sub.B, and V.sub.C from corresponding voltage and
current transformers operatively coupled to each of the A-, B- and
C-phases provided by the generator 12. Although illustrated as
being received via individual inputs, it should be understood that
the secondary current waveforms I.sub.A, I.sub.B, and I.sub.C and
secondary voltage waveforms V.sub.A, V.sub.B, and V.sub.C may be
received via a combination of phase-input current transformers,
current transformers, voltage transformers, and non-conventional
current and voltage sensors. In general, the secondary current
waveforms I.sub.A, I.sub.B, and I.sub.C and secondary voltage
waveforms V.sub.A, V.sub.B, and V.sub.C are processed to determine
whether an alarm and/or trip signal should be issued by the
protective relay 100.
[0039] More specifically, each of the secondary current and voltage
waveforms I.sub.A, I.sub.B, and I.sub.C and V.sub.A, V.sub.B, and
V.sub.C is further transformed into corresponding scaled sinusoidal
waveforms via current transformers 111-113 and voltage transformers
114-116 respectively, and resistors (not separately illustrated).
The scaled sinusoidal waveforms are filtered via (hardware) analog
low pass filters 121-126. A multiplexer 128 then selects each of
the filtered scaled sinusoidal waveforms, one at a time, and
provides the selected filtered scaled sinusoidal waveforms to an
analog-to-digital (A/D) converter 130. The A/D converter 130
samples and digitizes each of the selected filtered scaled
sinusoidal waveforms to form corresponding digitized signals
131-136. The corresponding digitized signals 131-136 are
representative of the A-, B- and C-phase secondary current and
voltage waveforms I.sub.A, I.sub.B, and I.sub.C and V.sub.A,
V.sub.B, and V.sub.C, respectively.
[0040] The corresponding digitized signals 131-136 are received by
a microcontroller 138 (or digital signal processor (DSP) or
personal computer ((PC))) for signal processing, where they are
digitally filtered via, for example, Cosine filters to eliminate DC
and unwanted frequency components. In the illustrated example of
FIG. 2a, the digital filtering is provided by digital band pass
filters (DBPFs) 141-146 where DBPFs 141 and 144 perform digital
filtering for digitized signals 131 and 134 to form filtered
digital signals 161 and 164 representative of the A-phase secondary
current and voltage waveforms I.sub.A and V.sub.A, where DBPFs 142
and 145 perform digital filtering for digitized signals 132 and 135
to form filtered digital signals 162 and 165 representative of the
B-phase secondary current and voltage waveforms I.sub.B and
V.sub.B, and where DBPFs 143 and 146 perform digital filtering for
digitized signals 133 and 136 to form filtered digital signals 163
and 166 representative of the C-phase secondary current and voltage
waveforms I.sub.C and V.sub.C. In an embodiment, the filtered
digital signals 161-166 are further converted into phasor form to
enable subsequent calculations by the microcontroller 138.
[0041] An indication input 180 is also provided to receive
generator operating indications such as, for example, pressure
transducer inputs indicative of generator operating parameters.
Other generator indications include cooling pressure, excitation or
field current, stator temperature, gearing temperature, ambient
temperature and the like.
[0042] The microcontroller 138 includes a generator operating
boundary function 148, and additional generator protection
functions 156 that comprise one or more protective elements. Using
the filtered digital signals 161-166 from the DBPFs 141-146, the
additional generator protection functions 156 performs one or more
typical protection functions. For example, the additional generator
protection functions 156 may include a differential protection
element, a stator ground fault protection element, a rotor ground
fault protection element, a motoring protection element, an
over-excitation protection element, a thermal protection element,
an under-frequency protection element, an over-current protection
element, an over-voltage protection element, and/or an out-of-step
protection element. Based on magnitudes and phase angles of the
phasors representing the filtered digital signals 161-166, the
additional generator protection functions 156 may actuate a trip
and/or an alarm indication.
[0043] As illustrated in FIG. 2b, the generator operating boundary
function 148 includes an A-phase P, Q calculator 150, a B-phase P,
Q calculator 152 and a C-phase P, Q calculator 154. Each of the
A-phase P, Q calculator 150, the B-phase P, Q calculator 152 and
the C-phase P, Q calculator 154 includes two inputs for receiving
corresponding filtered digital signals, and at least two outputs.
The generator operating boundary function 148 also includes a phase
sum P, Q calculator 160 and a curve function 158. The phase sum P,
Q calculator 160 is coupled to receive the outputs of the A-phase
P, Q calculator 150, the B-phase P, Q calculator 152 and the
C-phase P, Q calculator 154. The curve function 158 includes a
first (177) and second (178) inputs for receiving outputs from the
phase sum P, Q calculator 160, and a third input for receiving user
programmable inputs 182. While discussed in terms of P, Q
calculators, it should be understood that the generator operating
boundary function 148 may be implemented in one of any number of
suitable ways, for example, as software executed via operation of
the microcontroller 138.
[0044] More specifically, the A-phase P, Q calculator 150 includes
a first and a second input for receiving the filtered digital
signals 161 and 164, and a first and second output for providing an
A-phase P value 171 and an A-phase Q value 174, respectively, to
the phase sum P, Q calculator 160. During operation, the A-phase P,
Q calculator 150 calculates the A-phase P value 171 and the A-phase
Q value 174 based on corresponding A-phase secondary current and
voltage waveforms I.sub.A, V.sub.A 101, 104. The A-phase P value
171 represents a calculated active power operating point of the
synchronous generator 12, while the A-phase Q value 174 represents
a calculated reactive power operating point of the synchronous
generator 12 for the A-phase.
[0045] Similarly, the B-phase P, Q calculator 152 includes a first
and a second input for receiving the filtered digital signals 162
and 165, and a first and second output for providing a B-phase P
value 172 and a B-phase Q value 175, respectively, to the phase sum
P, Q calculator 160. During operation, the B-phase P, Q calculator
152 calculates the B-phase P value 172 and the B-phase Q value 175
based on B-phase secondary current and voltage waveforms I.sub.B,
V.sub.B 102, 105. The B-phase P value 172 represents a calculated
active power operating point of the synchronous generator 12 and
the B-phase Q value 175 represents a calculated reactive power
operating point of the synchronous generator 12 for the B-phase.
Likewise, the C-phase P, Q calculator 154 includes a first and a
second input for receiving the filtered digital signals 163 and
166, and a first and second output for providing a C-phase P value
173 and a C-phase Q value 176, respectively, to the phase sum P, Q
calculator 160. During operation, the C-phase P, Q calculator 154
calculates the C-phase P value 173 and the C-phase Q value 176
based on the C-phase secondary current and voltage waveforms
I.sub.C, V.sub.C 103, 106. The C-phase P value 173 represents a
calculated active power operating point of the synchronous
generator 12 and the C-phase Q value 176 represents a calculated
reactive power operating point of the synchronous generator 12 for
the C-phase.
[0046] Each of the A-phase P value 171, the A-phase Q value 174,
the B-phase P value 172, the B-phase Q value 175, C-phase P value
173 and the C-phase Q value 176 are received by the phase sum P, Q
calculator 160 where the A-phase P value 171, the B-phase P value
172 and the C-phase P value 173 are added together to form a P
value sum 177, and the A-phase Q value 174, the B-phase Q value 175
and the C-phase Q value 176 are added to form a Q value sum 178.
The P value sum 177 represents a sum of three-phase active power,
while the Q value sum 178 represents a sum of three-phase reactive
power.
[0047] As illustrated in FIG. 2c, positive-sequence calculators
181, 183 may further be included to provide positive-sequence
values for power calculation. In this embodiment, the following
equations may be utilized therein.
[0048] Positive-Sequence Voltage (Output 185)
V 1 = V A + a V B + a 2 V C 3 ( 1 ) ##EQU00001##
[0049] Positive-Sequence Current (Output 184)
I 1 = I A + a I B + a 2 I C 3 ( 2 ) ##EQU00002##
[0050] Positive-Sequence Apparent Power
S.sub.1=3Vconj(I.sub.1) (3)
[0051] Positive-Sequence Active Power (Output 177)
P.sub.1=real(S.sub.1) (4)
[0052] Positive-Sequence Reactive Power (Output 178)
Q.sub.1=imag(S.sub.1) (5)
[0053] Where:
a:=e.sup.j120.degree. (6)
[0054] Now referring concurrently to FIGS. 2a and 2b, as discussed
above, the curve function 158 includes a first input for receiving
the P value sum 177, a second input for receiving the Q value sum
178, and a third input for receiving user programmable inputs 182.
The curve function 158 further includes two outputs; a first output
for enabling transmission of a binary alarm bit and a second output
for enabling transmission of a binary trip bit. This arrangement
may further be adapted to include additional outputs for assertion
of alarm and/or tripping. For example, additional alarm bits may be
included for assertion of alarm conditions associated with the
field winding current limit, the armature current limit, the stator
end region heating limit, a generator motoring condition, or a
loss-of-field condition as will be discussed in further detail
below.
[0055] In an embodiment, the curve function 158 is mathematically
derived from, and is therefore representative of, a
manufacturer-provided set of specific capability curves. The curve
function 158 may be implemented as at least one set of derived
curve expressions (e.g., polynomial equations of the second order;
ax.sup.2+bx+c=0 or otherwise circle equations as discussed in
greater detail below), one or more look-up tables, one or more
linear equations, or equivalent means, collectively referred to
herein as generator safe operating boundary data expressions. Each
of the generator safe operating boundary data expressions may be
derived from power system data such as manufacturer-provided sets
of specific capability curves, SSSL curves, MEL curves, OEL curves
or the like, collectively referred to herein as generator safe
operating boundaries. As used herein, "power system data" may
further include power system parameters such as power system
equivalent impedance, power system component impedance, and the
like.
[0056] Thus, in an embodiment, the apparatus and method for
synchronous generator protection of the instant invention utilizes
derived curve expressions to perform a portion of the protective
functions of the protective relay 100. In another embodiment, the
apparatus and method for synchronous generator protection of the
instant invention may utilize MEL look-up tables, SSSL linear
equations, or any combination of capability curves, SSSL curves MEL
curves, and/or OEL curves suitably expressed in the form of
quadratic equations, circle equations, look-up tables, linear
equations, or equivalent means, to perform protective functions of
the protective relay 100.
[0057] In one embodiment, the curve function 158 is implemented as
a set of three quadratic equations derived from plotted P, Q
coordinates of associated manufacturer-provided capability curves.
In another embodiment, the curve function 158 is implemented
through separate circle equations which provide a graphical
representation of an estimation curve for the boundary for field
winding heating associated with the field winding current limit of
a generator capability curve; the boundary for armature heating
associated with the armature current limit of a generator
capability curve; and the boundary for stator core temperature
associated with the stator end region heating limit. It should be
understood however, that other implementations of the capability
curves may be used for the curve function 158 (e.g., look-up
tables). Further, it is contemplated that the curve function 158
may be derived from P, Q coordinates of SSSL, MEL curves, or OEL
curves. For example, a loss-of-field element characteristic may be
provided in relation to an SSSL curve, or the stator end region
heating limit curve of the capability curve, as will be discussed
in greater detail below.
[0058] While illustrated as functional blocks in FIG. 2a, the
microcontroller 138 may be implemented via one of any number of
suitable means. For example, in an embodiment, the microcontroller
138 may include a CPU, or a microprocessor, a program memory (e.g.,
a Flash EPROM) and a parameter memory (e.g., an EEPROM).
Alternatively, the microcontroller 138 may be implemented as a
field programmable gate array (FPGA), a digital signal processor
(DSP) or a PC-based platform, to name a few.
[0059] FIG. 3 is an exemplary set of generator capability curves
200 that may be provided by a synchronous generator manufacturer to
define the thermal, or heating, operating limits of the synchronous
generator 12. In the illustrated example, the set of generator
capability curves 200 represent actual capability curves for a 312
MW, 3600 RPM, inner-cooled turbine generator, where the active
power component "P" is expressed in MW on the horizontal axis and
the reactive power component "Q" is expressed in MVAR on the
vertical axis.
[0060] An overexcitation region (VARs are being supplied by the
synchronous generator 12), defined above the zero MVAR point on the
vertical axis, may also be referred to as lagging power factor
region. An underexcitation region (VARs are being consumed by the
synchronous generator 12), defined below zero MVAR on the vertical
axis, may also be referred to as the leading power factor region.
The capability curve 205 corresponds to a higher cooling system
hydrogen pressure (e.g., 3 kg/cm.sup.2) than the capability curve
203 (e.g., 2 kg/cm.sup.2). Accordingly, FIG. 3 illustrates that the
effectiveness of the cooling and hence the allowable generator
loading depends on the cooling pressure, and that the synchronous
generator 12 can provide increased power output when the cooling
system pressure is increased, provided that the prime mover has the
ability to provide the additional power.
[0061] For ease of discussion, FIG. 4 is the capability curve 205
of the exemplary set of generator capability curves 200. In
general, synchronous generators are rated in terms of maximum MVA
output at a specified voltage and power factor (pf) (e.g., 0.85
lagging) which they can carry continuously without overheating. The
active power output is limited by the prime mover capability to a
value within the MVA rating of the generator. The continuous
reactive power output capability is limited by the three factors;
an armature current limit, a field winding current limit and an end
region heating limit. The armature current limit associated with an
RI.sup.2 power loss is the maximum current that can be carried by
the armature without exceeding heating limitations. The field
current limit is associated with an R.sub.fdi.sub.fd.sup.2 power
loss. The localized heating in the end region of the stator imposes
a third limit on the synchronous generator 12 which affects the
capability of the generator in the underexcited condition.
[0062] Referring to FIG. 4, for each cooling pressure, a first
curve portion 206 of the capability curve 205 represents a boundary
for field winding heating associated with the field winding current
limit. This is also generally referred to as the rotor current
limit. The rotor-current limit on the generator field current
generally results from copper power losses in the rotor winding. A
second curve portion 210 of the capability curve 205 represents a
boundary for armature heating associated with the armature current
limit. The armature current limit on the generator field current
generally results from stator copper power losses, wherein there is
generally a maximum current that a generator can carry continuously
without exceeding the allowable operating temperature. A third
curve portion 208 of the capability curve 205 represents a boundary
for a stator core temperature associated with the stator end region
heating limit. This is generally referred to as the stator end
heating limit.
[0063] An interior region 240 bounded by the first, second and
third curve portions 206, 210, 208 is referred to as a safe
operation region 240 indicating normal generator operation, while
an exterior region 242 outside of the interior region is referred
to as an unsafe operation region 242 indicating abnormal generator
operation.
[0064] Typically, the manufacturer-provided generator capability
curves are utilized by a power station operator to determine the
operational capability of an associated synchronous generator and
to determine whether additional MW can be obtained from the
synchronous generator under various conditions. For example,
referring to FIG. 4, when operated at a lagging power factor (pf)
of 0.9 (point 218), the synchronous generator 12 will generate
312.5 MW of active power and 150 MVAR of reactive power. When
operated at a leading pf of 0.9 (point 226), the synchronous
generator 12 will generate 330 MW of active power and absorb 112.5
MVAR of reactive power. When operated at a pf of 1.0 (point 222),
the synchronous generator 12 will generate a maximum of 345 MW of
active power and no MVAR of reactive power.
[0065] An SSSL curve 244 and an MEL curve 246 are also illustrated
in FIG. 4. The SSSL curve 244 represents a power limit to maintain
system stability. The SSSL curve will vary with the synchronous
generator and with the power system connected, as well as with
voltage.
[0066] All synchronous generators connected to the power system
operate at the same average speed. The generator speed governors
maintain the machine speed close to its nominal value. There is a
balance between generated and consumed active power under normal
power system operating conditions. Random changes in load and
system configuration constantly take place and impose small
disturbances to the power system. The property of a power system to
keep the normal operating condition under these small slow changes
of system loading is generally known as steady-state stability or
system stability for small perturbations.
[0067] For the two-machine power system the active power transfer
P.sub.e is given by:
P e = E q E s X d + X s sin .delta. ( 7 ) ##EQU00003##
[0068] wherein the generator internal voltage and synchronous
reactance are E.sub.q and X.sub.d respectively; the power system
voltage and reactance are E.sub.s and X.sub.s respectively; and the
system power angle .delta. is the angle between E.sub.q and
E.sub.s.
[0069] Referring back to FIG. 4, the center position and radius of
the SSSL circle are expressed by the following equations:
Center ( P , Q ) = 0 , V t 2 2 ( 1 X s - 1 X d ) ( 8 ) Radius = V t
2 2 ( 1 X d - 1 X s ) ( 9 ) ##EQU00004##
[0070] Wherein V.sub.t is the generator terminal voltage. Typically
when the power system is strong (X.sub.s is low) the SSSL locus is
outside the generator capability curve. However, on weak systems,
the manual SSSL can be more restrictive than the generator
capability in the underexcited region.
[0071] Under automatic operation, the automatic voltage regulator
(AVR) rapidly varies the field current in response to system
operating conditions. This changes the maximum value of the power
angle curve upwards or downwards as required by the system. This
dynamic response improves the SSSL as compared to that resulting
from manual regulator operation. The effect of AVR on SSSL depends
on the voltage regulator gain, the regulator time constant and the
field time constant.
[0072] MEL is a control function included in the automatic voltage
regulator that acts to limit reactive power flow into the
generator. During normal operation, the AVR keeps generator voltage
at a preset value. When system conditions require the generator to
absorb reactive power in excess of the MEL set point, the MEL
interacts with the AVR to increase terminal voltage until reactive
power inflow is reduced below the setting. The MEL curve 246
represents a boundary below which the MEL included in the AVR of
the synchronous generator 12 operates to restrict generator
reactive power inflow. The MEL curve 246 is situated just above the
SSSL curve 244.
[0073] Overexcitation limiter (OEL) is a control function included
in the AVR that protects the generator from overheating resulting
from prolonged field overcurrent. OEL detects the field-overcurrent
condition and acts with time delay to ramp down the excitation to a
preset value. The OEL operating characteristic (not shown in FIG.
4) plots as a line in the P-Q plane, placed below the field winding
current limit curve 206.
[0074] The generator safe operating boundary expressions may be
derived from generator data and/or power system data (e.g., in
relation to the stator end region heating limit boundary 208, MEL
curve 246, OEL curve (not shown), or SSSL curve 244). For example,
as will be discussed in greater detail below, loss-of-field
protection may be provided by situating a generator safe operating
boundary expression in relation to the stator end region heating
limit boundary 208, or SSSL curve 244.
[0075] Referring again to FIG. 2a, the user programmable inputs 182
include pre-programmed user inputs that may be selected/set during
commissioning of the protective relay 100. In an embodiment, each
of the user programmable inputs 182 corresponds to one of a number
of sets of derived curve expressions selectable by the
microcontroller 138 upon occurrence of specified generator
operating conditions. However, other selection arrangements are
contemplated.
[0076] As described above, each set of curve expressions is derived
from plotted P, Q coordinates of a manufacturer-provided capability
curve. For example, one set of curve expressions is derived from
the P, Q coordinates of the first, second and third curve portions
206, 208, 210 of the capability curve 205. Similarly, a different
set of curve expressions may be derived from the P, Q coordinates
of the capability curve 203.
[0077] In general, during relay operation, the microcontroller 138
selects a particular set of derived curve expressions for the curve
function 158 based on its user programmable input(s) 182, the relay
operating conditions determined from the measured secondary
currents and voltages, and the generator operating indications
received via the indication input 180. As the generator operating
conditions change, so do the sets of derived curve expressions
utilized by the microcontroller 138 when performing the generator
operating boundary function 148. For example, using
manufacturer-provided capability curves as a basis, the user may
specify, via the user programmable inputs 182, that the
microcontroller 138 utilize a first set of derived curve
expressions upon detecting a generator operating pressure of 3
Kg/cm.sup.3, and utilize a second set of derived curve expressions
upon detecting a generator operating pressure of 2 Kg/cm.sup.3.
Other generator operating conditions such as generator temperature
measurements from associated temperature transducers, etc., may be
used as a basis for the user programmable inputs 182 and subsequent
microcontroller selection of derived curve equation sets.
[0078] Although illustrated as including only one user programmable
input 182, it is contemplated that the curve function 158 may
include more or less user programmable inputs 182, depending on the
implementation. Further, although illustrated as one curve, the
curve function 158 may include a number of sets of curve
expressions derived from multiple manufacturer-provided capability
curves, and therefore represent multiple operating limits at
different cooling system pressures.
[0079] Additionally, one curve function may be assigned to
determine an alarm bit output, and another curve function may be
assigned to determine a trip bit output. Assertion of an alarm
and/or trip output bit may be delayed via operation of one or more
timers such as a first timer 191 and a second timer 192. For
example, an output of a comparison of the P value sum 177 and the Q
value sum 178 to one set of derived curve expressions may actuate
an alarm action after a 10 second timeout of the timer 191, while
an output of a comparison of the P value sum 177 and the Q value
sum 178 to another set of derived curve expressions may actuate a
trip action after a 0.2 second timeout of the timer 192.
[0080] Thus, during relay operation, in addition to providing
protective functions such as differential protection, ground fault
protection, etc., illustrated as the additional generator
protection functions 156, the microcontroller 138 compares the P
value sum 177 and the Q value sum 178 to a selected set(s) of
derived curve expressions to determine whether operating conditions
of the synchronous generator 12 are inside or outside of the
generator safe operating boundaries, and to actuate subsequent
alarm and/or trip actions when warranted. Such P and Q value sums
177, 178 reflect the "P, Q operating point" of the synchronous
generator 12.
[0081] FIG. 5a is a flowchart of a method 300 for providing
synchronous generator protection in the power system 10, according
to an embodiment of the invention. In general, for each
manufacturer-provided generator capability curve, the method 300
includes deriving sets of curve expressions or curve elements from
any of generator capability curve data, generator impedance
characteristic, and/or power system data for use by the protective
relay 100 during operation. While implemented as sets of derived
curve expressions, it should be understood that the expressions may
be linear, quadratic or otherwise circle equations, and that other
generator safe operating boundary data expressions (e.g., look-up
tables) are contemplated. Other generator safe operating boundaries
(e.g., situated in relation to SSSL curves, MEL curves, or OEL
curves) may be used as a basis for deriving the generator safe
operating boundary data expressions.
[0082] The method 300 begins when a number of sets of curve
elements or expressions are derived from power system data such as
generator capability curve data, generator impedance
characteristic, and/or power system parameters (step 303). For
example, close approximations of the first, second and third curve
portions 206, 210, 208 of the capability curve 205 of FIG. 4 can be
expressed as one set of derived curve expressions. The set of
derived curve expressions may be calculated using a curve-fitting
algorithm such as one available in Matlab.RTM. (or similar
curve-fitting algorithm) along with a number of plotted (P, Q)
coordinates of the capability curve 205.
[0083] For example, Table 1 illustrates a number of (P, Q)
coordinates of the capability curve 205 that may be used to derive
one set of curve expressions.
TABLE-US-00001 TABLE 1 Curve 1 (206) Curve 2 (210) Curve 3 (208) P
(MW) Q (MVAR) P (MW) Q (MVAR) P (MW) Q (MVAR) 0 212.5 312.5 150 0
-168 137.5 200 343.7 50 100 -162.5 200 187.5 345 0 237.5 -137.5
312.5 150 343.7 -50 329.3 -108.2 329.3 -108.2
[0084] Where, in FIG. 4:
[0085] 0 MW and 212.5 MVAR is represented by reference number
212,
[0086] 137.5 MW and 200 MVAR is represented by reference number
214,
[0087] 200 MW and 187.5 MVAR is represented by reference number
216,
[0088] 312.5 MW and 150 MVAR is represented by reference number
218,
[0089] 343.7 MW and 50 MVAR is represented by reference number
220,
[0090] 345 MW and 0 MVAR is represented by reference number
222,
[0091] 343.7 MW and -50 MVAR is represented by reference number
224,
[0092] 329.3 MW and -108.2 MVAR is represented by reference number
226,
[0093] 237.5 MW and -137.5 MVAR is represented by reference number
228,
[0094] 100 MW and -162.5 MVAR is represented by reference number
230, and
[0095] 0 MW and -168 MVAR is represented by reference number
232.
[0096] In one embodiment, using Table 1, a set of three derived
curve expressions (10), (11) and (12) approximating the first,
second and third curve portions 206, 210, 208 can be derived from
the plotted (P, Q) coordinates reflected in Table 1, where the
active power component "P" is expressed as a function of the
reactive power component "Q". In this embodiment, the curve
expressions are in the form of a set of three quadratic equations
derived from plotted P, Q coordinates of associated
manufacturer-provided capability curves. Each quadratic equation
provides a graphical representation of an estimation curve for the
boundary for field winding heating associated with the field
winding current limit (Curve 1); the boundary for armature heating
associated with the armature current limit (Curve 2); and the
boundary for a stator core temperature associated with the stator
end region heating limit (Curve 3). Other arrangements are
possible.
[0097] Curve 1 Quadratic Equation:
P(Q)=-0.0882Q.sup.2-19.2898Q-726.1124 (10)
[0098] Curve 2 Quadratic Equation:
P(Q)=-0.0011Q.sup.2-0.0234Q+340.1911 (11)
[0099] Curve 3 Quadratic Equation:
P(Q)=0.0404Q.sup.2+10.6301Q-373.5395 (12)
[0100] FIG. 6 is a generator capability curve 400 drawn based on
the set of derived curve expressions defined by equations (10),
(11) and (12), according to an embodiment of the invention. For
each of the three derived curve expressions of FIG. 6, the active
power component is expressed as a function of the reactive power
component. Similarly, additional sets of three curve expressions
may be derived from other manufacturer-provided capability curves.
Each set represents generator operating (thermal) limits at a
specific cooling pressure. As discussed above, approximations of
the SSSL curves and MEL curves may also be derived in a suitable
form, and then used by the protective relay 100 during generator
operation.
[0101] In another embodiment, a set of three derived curve
expressions (13), (17) and (21) approximating the first, second and
third curve portions 206, 210, 208 can be derived from the plotted
(P, Q) coordinates (e.g., reflected in Table 1), where the active
power component "P" is expressed as a function of the reactive
power component "Q". In this embodiment, the curve expressions are
in the form of a set of three circle equations derived from plotted
P, Q coordinates of associated manufacturer-provided capability
curves. Each circle equation provides a graphical representation of
an estimation curve for the boundary for field winding heating
associated with the field winding current limit (Curve 1); the
boundary for armature heating associated with the armature current
limit (Curve 2); and the boundary for a stator core temperature
associated with the stator end region heating limit (Curve 3).
Other arrangements are possible.
[0102] More specifically, the following circle equation (equation
(13)) may provide the graphical representation of the estimation
curve for the boundary for field winding heating associated with
the field winding current limit (Curve 1) as illustrated in FIG.
7.
Curve 1 Circle Equation for Field Winding Current Limit:
[0103] S ( .beta. ) = R e i .beta. + i C for .rho. .ltoreq. .beta.
.ltoreq. .pi. 2 ( 13 ) ##EQU00005##
[0104] Where: [0105] R is the radius of the circle [0106] C is the
center of the circle [0107] .rho. is the circle lower limit
[0108] For Curve 1 the following equations (equations (14), (15),
and (16)) may be solved in order to obtain the values for R, C, and
.rho..
Rcos .rho.=p.sub.1 (14)
Rsin .rho.+C=q.sub.1 (15)
R+C=q.sub.0 (16)
[0109] The following circle equation (equation (17)) may provide
the graphical representation of the estimation curve for the
boundary for armature heating associated with the armature current
limit (Curve 2) as illustrated in FIG. 8:
[0110] Curve 2 Circle Equation for Armature Current Limit:
S(.beta.)=Re.sup.i.beta.+iC for -.alpha.-.beta..ltoreq..phi.
(17)
[0111] Where: [0112] R is the radius of the circle [0113] C is the
center of the circle [0114] .phi. is the circle upper limit that
corresponds to the minimum lagging power factor [0115] -.alpha. is
the circle lower limit that corresponds to the minimum leading
power factor
[0116] For Curve 2 the following equations (equations (18), (19),
and (20)) may be solved in order to obtain the values for R,
.alpha., and .phi..
R=S.sub.nom (18)
.phi.=cos.sup.-1(PF.sub.Lag)=cos.sup.-1(P.sub.1/S.sub.nom) (19)
.alpha.=cos.sup.-1(PF.sub.Lead)=cos.sup.-1(P.sub.2/S.sub.nom)
(20)
[0117] Where: [0118] PF.sub.Lag is the minimum lagging power factor
[0119] PF.sub.Lead is the minimum leading power factor [0120]
S.sub.nom is the generator rated capacity
[0121] The following circle equation (equation (21)) may provide
the graphical representation of the estimation curve for the
boundary for a stator core temperature associated with the stator
end region heating limit (Curve 3) as illustrated in FIG. 9:
[0122] Curve 3 Circle Equation for Stator End Region Heating
Limit:
S ( .beta. ) = R e i .beta. + i C for 3 2 .pi. .ltoreq. .beta.
.ltoreq. - .gamma. ( 21 ) ##EQU00006##
[0123] Where: [0124] R is the radius of the circle [0125] C is the
center of the circle [0126] -.gamma. is the circle upper limit
[0127] For Curve 3 the following equations (equations (22), (23),
and (24)) may be solved in order to obtain the values for R, C, and
.gamma..
Rcos .gamma.=p.sub.2 (22)
C-Rsin .gamma.=q.sub.2 (23)
C-R=q.sub.3 (24)
[0128] Using circle equations (13), (17) and (21), a generator
capability curve may be drawn similar to that shown in FIG. 6. As
discussed above, approximations of the SSSL curves, MEL curves and
OEL curves or user-entered curves may also be derived in a suitable
form using circle equation (21). For example and as will be
discussed in greater detail below, using the circle equation of
Curve 3 (equation (21)), the estimation curve for a loss-of-field
protection element curve such that it is situated in relation to
the stator end region heating limit curve, or in relation to SSSL
may also be derived. In yet another embodiment, the operating
region defined in order to provide for loss-of-field protection is
located below a loss-of-field element characteristic (situated with
respect to the stator end region heating limit curve, or in
relation to an SSSL curve), and between two active power elements
vertical straight lines.
[0129] In one embodiment, as shown in the P-Q plane of FIG. 10, a
loss-of-field protection characteristic is provided, including a
loss-of-field element characteristic and two active power element
vertical straight lines. The use of the embodiment of FIG. 10 with
the aforementioned apparatuses, systems and methods of the present
invention is generally beneficial where the SSSL characteristic is
outside the capability curve. In this arrangement, the
loss-of-field element characteristic is set coinciding with the
stator end heating limit curve, and situated above the SSSL curve.
Situating the loss-of-field element in this manner allows the
capability curve to protect the generator from stator end core
heating. This arrangement further allows using the full generator
capability to absorb reactive power, beyond the MEL setting.
[0130] In another embodiment, as shown in the P-Q plane of FIG. 11,
another loss-of-field protection characteristic is provided. The
use of the embodiment of FIG. 11 with the aforementioned
apparatuses, systems and methods of the present invention is
generally beneficial where the SSSL characteristic is inside the
capability curve, which may occur in weak power systems. In this
arrangement, the loss-of-field element characteristic is situated
above the SSSL curve, and also inside the capability curve.
However, in comparing the P-Q planes of FIGS. 10 and 11, the
loss-of-field element of FIG. 11 is situated generally closer to
the SSSL curve. Situating the loss-of-field element in this manner
limits the amount of reactive power that the generator can
absorb.
[0131] In the arrangements of FIGS. 10 and 11, the loss-of-field
element characteristic along with the capability curve (comprising
curves representing a rotor current limit and armature current
limit) define a generator normal operation zone, an alarming zone
and a protection zone. The area bounded by the loss-of-field
element characteristic and the capability curve defines the
generator normal operation zone, whereas the area bounded by the
two active power elements vertical straight lines and the
loss-of-field element characteristic defines the protection zone.
It is to be noted that the area outside the area bounded by the
loss-of-field element and the capability curve defines an alarming
zone as discussed in detail above. As discussed in full detail
above, when the generator operating condition falls within the
protection zone and/or the alarming zone, the apparatus of FIG. 2a
is adapted to assert an alarm and/or trip signal.
[0132] The active power elements serve as blinders which restrict
coverage along the P axis of the P-Q plane. The left-side active
power element may be set to any value. In one embodiment, the left
side active power element is set to coincide with the Q axis. The
right-side active power element may be set to any value. For
example, the right-side active power element may be set such that
it adapts to the generator load condition. In another example, the
right-side active power element may be set to the measured
pre-disturbance active power, plus 20% of the generator rated
active power. In yet another example, the upper limit of the
right-side active power element may be set to the generator MVA
rating or, alternatively the turbine MW rating.
[0133] In yet another embodiment, the generator loss-of-field
protection element may further include an undervoltage element or
curve (not shown). In this arrangement, the undervoltage element
operates to accelerate a trip and/or alarm assertion when a low
voltage condition indicates that the system may collapse. For
example, an undervoltage element may be provided and set to 0.8-0.9
of the generator nominal voltage. Once the generator voltage falls
below this value, a trip and/or alarm signal is asserted.
[0134] In one embodiment, which may be referred to as the manual
method, the techniques described herein relating to deriving a set
of expressions to determine the generator capability curve and
loss-of-field protective element characteristic may be performed by
a protective relay automatically. In this embodiment, the user
enters power system data that includes data based on the capability
curve and the user-defined loss-of-field element characteristic.
For example, the user may enter two coordinates corresponding with
the rotor current limit 206 such as (p.sub.0, q.sub.0) 212 and
(p.sub.1, q.sub.1) 218 of FIG. 4. For the loss-of-field element,
the user may enter coordinates on the user-defined loss-of-field
element, and select between a straight-line configuration and a
curved-line configuration. The straight-line configuration may
include one or more straight-line approximations. Also, the
curved-line configuration may include one or more curved-line
approximations. For example, the user-defined loss-of-field element
may correspond with the stator end heating limit 208 of the
capability curve 205, and the user may enter coordinates on this
curve such as (p.sub.3, q.sub.3) 232 and (p.sub.2, q.sub.2) 226 of
FIG. 4. Alternatively, the user-defined loss-of-field element may
lie above the stator end heating limit 208 as illustrated in FIG.
11. From these entered coordinates, the generator safe operating
boundary data expressions may be derived by techniques described
above from the entered power system data.
[0135] In another embodiment, which may be referred to as the
automatic method, the user need not enter a user-defined
loss-of-field element. However, the user does enter power system
data including generator manufacturer data and power system
parameters. The generator manufacturer data includes coordinates of
points on the capability curve, as described above and in
conjunction with FIG. 4 and Table 1, and the generator impedance.
The power system parameters includes the equivalent system
impedance. From this, the capability curve and the loss-of-field
element characteristic are derived automatically, for example, by a
protective relay, as further described herein. Thus, the generator
safe operating boundary data expressions are derived from a
plurality of power system data.
[0136] Referring again to FIG. 5a, next a number of sets of derived
curve expressions (e.g., quadratic, circle or any other suitable
equations) are provided to the protective relay 100 for selection
and use by the microcontroller 138 when performing the generator
operating boundary function 148 (step 304). In general, during
relay operation, the microcontroller 138 determines which set of
derived curve expressions should be used as the curve function 158.
Such "selected sets of derived curve expressions" may vary
depending on previously entered user programmable inputs 182 (step
311), on generator operating indications (step 305), or on
generator terminal voltage and/or stator current (step 306). For
example, if a generator operating indication is determined to be 2
Kg/cm.sup.3, a first set of derived curve expressions derived from
a first generator-manufacturer capability curve is used, and if a
generator operating indication is determined to be 3 Kg/cm.sup.3, a
second set of derived curve expressions derived from a second
generator-manufacturer capability curve is used. The selected sets
of derived curve expressions may also vary depending on the
configuration of the generator operating boundary function 148
(e.g., multiple user programmable inputs 182, multiple sets of
derived curve expressions used to provide a binary output to
actuate an alarm and/or a trip bit, etc.)
[0137] Among other things, both secondary voltage and current
waveforms V.sub.A, V.sub.B, V.sub.C, I.sub.A, I.sub.B, and I.sub.C
are processed by the protective relay 100 to form the P value sum
177 and the Q value sum 178 as described in connection with FIG.
2a. The P and Q value sums 177 and 178 determine the P, Q operating
point of the synchronous generator 12 at a particular moment in
time. Generator cooling pressures and the like however, are
determined from generator operating indications received via the
indication input 180 (see FIG. 2a). In one embodiment, pressure
transducers may provide generator cooling gas pressure measurements
to the microcontroller 138 via the indication input 180. Other
generator operating indications may be used such as, for example,
excitation or field current, stator temperature, gearing
temperature, ambient temperature and the like. With the user
programmable inputs, generator operating indications, and/or
generator terminal voltage and/or stator current, the
microcontroller 138 calculates the P and Q value sums and
determines the P-Q operating point of the synchronous generator.
(step 308).
[0138] After determining the P-Q operating point, the
microcontroller 138 adapts the protective element characteristics
of the relay to the generator operating conditions (step 309). The
microcontroller 138 compares the P, Q operating point of step 308
to protective element characteristics adapted in step 309 to
determine whether the P, Q operating point falls within the safe
operation region 240 or the unsafe operation region 242 of a
corresponding curve approximated by the selected set of derived
curve expressions.
[0139] FIG. 5b illustrates an embodiment of a method for
determining whether an alarm or trip condition should be asserted.
If the P, Q operating point falls within the safe operation region
240 of the curve (e.g., as defined by the selected set of derived
curve expressions, etc.), the microcontroller 138 concludes that
the synchronous generator 12 is operating within its normal limits
(i.e., the normal operating region) and no action is taken.
[0140] Another example of a generator normal operation zone is
further shown as the unshaded region of FIG. 12. An example of a
safe operating condition is point P.sub.A, Q.sub.A in FIG. 13. It
is to be noted that this safe operation region is bounded by the
loss-of-field element characteristic along with the capability
curve (comprising curves representing a rotor current limit and
armature current limit), and it is also bounded by an active power
element characteristic, which coincides with the Q axis of the P-Q
plane.
[0141] Referring back to FIG. 5 b, if the P, Q operating point is
determined to fall within the unsafe operation region 242 (e.g., as
defined by the selected set of derived curve expressions, etc.),
the microcontroller 138 concludes that the synchronous generator 12
is not operating within its safe limits and causes an action.
Another example of an alarming zone is further shown as the shaded
region of FIG. 12, which corresponds to an armature current limit
violation, a rotor current limit violation, a motoring condition, a
loss-of-field condition or otherwise an under excitation condition.
FIG. 13 further illustrates the protection zone based on a
loss-of-field element, wherein a trip signal is asserted if an
operating condition would fall therein.
[0142] In the case of abnormal generator operation, the action may
include actuating an audible alarm to indicate the unsafe operating
conditions, actuating a trip signal to remove the synchronous
generator 12 from service, notifying the power station operator via
a mobile text message or a computer terminal display message, etc.
Other notification or remedial actions are contemplated.
[0143] More specifically, in one example, the alarm characteristic
in the P-Q plane may be formed by the upper and right side branches
of the capability curve, by the loss-of-field element
characteristic, and by an active-power characteristic that
coincides with the Q axis. In one arrangement, the SSSL
characteristic may be situated outside the capability curve.
Accordingly, as shown in FIG. 12 and similar to FIG. 10, the alarm
characteristic fully coincides with the generator capability curve.
Depending on the limit violated by the generator operating point
(P,Q), the alarm element issues one of the following alarms,
Armature-Current Limit Violation; Rotor-Current Limit Violation;
Loss-of-field/Underexcitation Condition or Motoring Condition
[0144] In yet another embodiment, when the SSSL characteristic may
be situated inside the capability curve, as shown in FIG. 11, the
lower side of the alarm characteristic lies inside the capability
curve, coinciding with the loss-of-field element characteristic.
Depending on the limit violated by the generator operating point
(P,Q), the alarm element issues one of the following alarms,
Armature-Current Limit Violation; Rotor-Current Limit Violation;
Loss-of-field/Underexcitation Condition or Motoring Condition.
[0145] FIG. 13 illustrates yet another embodiment, comprising both
a loss-of-field protection characteristic and a capability curve
violation alarming characteristic. In this embodiment, an alarm
signal is asserted if an operating condition would fall out of the
"generator normal operation zone". Additionally, a trip signal is
asserted after a time delay if an operating condition would fall
within the "relay protection zone".
[0146] For example, referring concurrently to FIGS. 5b, 12 and 13,
if there is an armature current limit violation, a rotor current
limit violation, a motoring condition, a loss-of-field condition or
otherwise an under excitation condition, an alarm is asserted (for
example, a rotor current limit violation is shown as point
(P.sub.B, Q.sub.B) of FIG. 13). Additionally, referring
concurrently to FIGS. 5b and 13, if there is a loss-of-field
condition or otherwise an under excitation condition, a trip signal
may also be asserted to trip the associated generator and/or field
breaker (e.g., shown as point (P.sub.C, Q.sub.C) of FIG. 13).
[0147] The present method may be implemented as a computer process,
a computing system or as an article of manufacture such as a
computer program product or computer readable medium. The computer
program product may be a computer storage media readable by a
computer system and encoding a computer program of instructions for
executing a computer process. The computer program product may also
be a propagated signal on a carrier readable by a computing system
and encoding a computer program of instructions for executing a
computer process.
[0148] In one embodiment, the logical operations of the present
method are implemented (1) as a sequence of computer implemented
acts or program modules running on a computing system and/or (2) as
interconnected machine logic circuits or circuit modules within the
computing system. The implementation is a matter of choice
dependent on the performance requirements of the computing system
implementing the invention. Accordingly, the logical operations
making up the embodiments of the present invention described herein
are referred to variously as operations, structural devices, acts
or modules. It will be recognized by persons skilled in the art
that these operations, structural devices, acts and modules may be
implemented in software, in firmware, in special purpose digital
logic, and any combination thereof without deviating from the
spirit and scope of the present invention as recited within the
claims attached hereto.
[0149] While this invention has been described with reference to
certain illustrative aspects, it will be understood that this
description shall not be construed in a limiting sense. Rather,
various changes and modifications can be made to the illustrative
embodiments without departing from the true spirit, central
characteristics and scope of the invention, including those
combinations of features that are individually disclosed or claimed
herein. Furthermore, it will be appreciated that any such changes
and modifications will be recognized by those skilled in the art as
an equivalent to one or more elements of the following claims, and
shall be covered by such claims to the fullest extent permitted by
law.
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